VEGETATION CONTROLS ON CHANNEL COMPLEXITY IN SEMI-ARID URBAN STREAMS: A CASE STUDY OF MESA CREEK, COLORADO

Urbanizing streams are categorized using the urban stream response framework, which allows researchers, stakeholders, and managers to predict the degree of channel change and timescale of response. However, semi-arid urban streams do not fit neatly into the typical urban stream response. Here, we study the geomorphic characteristics of Mesa Creek, a semi-arid urban stream located in Colorado Springs, Colorado. We used aerial photos, sediment grain size analysis, channel geometry, vegetation surveys, and hydraulic modeling to characterize historic changes, grain size evolution in space and time, and channel complexity. Grain size analysis shows an increase in grain size distribution over the past 100 years that could be due to either greater streamflow, narrower channels, channel steepening or a combination thereof – aerial photos and historic accounts show evidence of all three. Aerial photos also indicate Mesa Creek was previously a braided channel that now alternates between single thread and anastomosing; this planform change was coincident with a heightened baseflow caused by golf course construction and irrigation. HEC-RAS 2D flow models show that baseflow changes also lead to more unpredictable flooding patterns in flashy storm events and to an increase in shear stress during those storms, further supporting field evidence for enhanced shear stress in modern versus historic deposits. Modern grain size distributions differ between single and multi-thread planforms and suggests that either grain size controls channel type or vice-versa. Vegetation growth and densification coincident with the loss of braided planforms led to vegetation-fixed channel geometries. These vegetation-controlled reaches show significantly greater channel and bedload complexity than non-vegetated reaches. Overall, our results showcase a unique response to urbanization in a semi-arid stream, and indicate that in the absence of dynamic braided channels, stable vegetation can provide channel complexity and associated habitat diversity.